There are ongoing investigations into joint transmission multi-user multi-input multi-output (JT-MU-MIMO), which combines joint transmission (JT) with multi-user multi-input multi-output (MU-MIMO). Joint transmission is implemented using transmission points (referred to as TPs below) that have been arranged at different locations, and in MU-MIMO, spatial multiplexing is used to transmit downlink signals to plural communication terminals (referred to as user equipment (UE) below). In JT-MU-MIMO, in order to reduce downlink signal interference between UEs receiving spatially multiplexed downlink signals, at the TPs, transmission signals are multiplied by transmission weights acquired using zero-forcing (ZF) or an orthogonalization technique such as block-diagonalization.
Differences in propagation delays arise between TPs and UEs as a result of differing propagation distances between each TP and the UEs. For example, when a downlink signal is transmitted from a second TP to a UE while a downlink signal is being transmitted from a first TP to the UE, the following occurs. If the distance between the second TP and the UE is longer than the distance between the first TP and the UE, then the downlink signal transmitted from the second TP is received by the UE at a delay with respect to the downlink signal transmitted from the first TP.
In such cases, the received phase of the downlink signal transmitted from the second TP is rotated in the frequency domain with respect to the received phase of the downlink signal transmitted from the first TP. Namely, the downlink signals transmitted from the two TPs are received at the UE with a different phase difference at each frequency, and since the interference between the downlink signals from the two TPs is frequency dependent, the optimal transmission weights are frequency dependent.
In order to acquire optimal frequency-dependent transmission weights, in other technology, for example, the frequency band is divided into predetermined band segments and a transmission weight is computed for each segment. However, processing load increases as the number of segments increases.
There is also technology in which transmission weights are first computed without increasing the number of segments, namely, without narrowing the given bands in each segment division, and then frequency intervals are narrowed and transmission weights interpolated. However, in such technology, it is difficult to interpolate transmission weights with appropriate values approximating the optimal transmission weights when there are large differences in propagation delays.